MySpace and Facebook lead the pack with 9.2% and 8.2% respectively. MySpace served up 30 billion impressions to 64 million unique visitors while Facebook served up 26.8 billion impressions to 67 million uniques.

"Over the past few years, social networking has become one of the most popular online activities, accounting for a significant portion of the time Internet users spend online and the pages they consume," said Jeff Hackett, comScore senior vice president. "Because the top social media sites can deliver high reach and frequency against target segments at a low cost, it appears that some advertisers are eager to use social networking sites as a new advertising delivery vehicle."